In recent years, Ethernet (registered trademark) has been increasingly employed in networks in industrial fields, for example, in automobiles, trains or factories. Frames used in industrial Ethernet networks generally include a control system frame, which is a frame that handles device control information, and a frame that handles information other than that on the control system, for example, video or voice. The control system frame needs transferring with low latency to meet a strict latency request for high speed and high reliability. Such a control system frame is hereinafter referred to as a “low-latency frame”. On the other hand, a frame that handles other information than that on the control system is hereinafter referred to as “a normal-latency frame” that should meet a less strict latency request.
Industrial Ethernet networks can include applications using different line speeds and transfer apparatuses including their ports having different line speeds. When the transfer apparatus transfers a frame from a port with a low line speed to a port with a high line speed, the transfer apparatus needs to store frame data so that underrun does not occur when the frame input from the low speed port is output. Patent Literature 1 discloses a technique for starting to transfer frames to a high speed port either after completing frame reception at a low speed input port or after waiting for a length of time ensuring no occurrence of underrun of frames being transferred from the low speed port to the high speed port.
In addition, Institute of Electrical and Electronic Engineers (IEEE) 802.3br proceed with standardization of media access control (MAC) employing interspersing express traffic (IET) technology for reducing transfer latency time of low-latency frames in transfer apparatuses. For the IET, when a low-latency frame becomes transferable during transfer of a normal-latency frame, interrupt transfer of the low-latency frame is performed as the ongoing transfer of the normal-latency frame is suspended and the normal-latency frame is divided to the extent that satisfies a minimum frame length of Ethernet and a post-division minimum frame length specified to be greater than the minimum frame length of Ethernet. A transfer apparatus including the MAC employing the IET can reduce transfer latency time of low-latency frames in the industrial Ethernet networks.